Hello all, if I may ask...

I have a 99 Polaris Sportsman 500 that I acquired not running and very cheap as a result. I have cleaned it up, did some engine CSI to determine no compression and some mechanical lockup inside, took the engine out and top end off. Piston has several pieces broken off edge in one spot. Rings and cylinder substantially destroyed of course. I have the cylinder off of the case, and have thoroughly cleaned all small pieces out of the case. I don't think it was ran like this but for a second. The previous owner said he was riding it and the wheels locked up, he skid to a stop and it never ran again. No visible damage in lower end that I can see. I have rinsed it all out well. All gears mesh correctly, no broken teeth, timing gears and chain all seem ok, rod has very very little bit of side to side play, crank is rotating smoothly and no play can be felt, crank seals were not leaking, nothing seems sloppy anywhere. The edge of the inside of cylinder head, adjacent the intake vales does look like the piston pieces hit it and slightly mushroomed it there inside the cylinder. I noticed the head bolt alignment dowels (knock tubes) looked slightly bent. The valves don't look bad but I don't know how to check if they are bent, and if they are seating and sealing? The cam has no excessive wear, rockers look ok. No missing parts that I can tell.

I now have on hand 2 new alignment dowels, a new cylinder, piston, rings, and gaskets. My question, before I put this stuff back together... I am leaning towards taking my chances with the lower end, it just looks all ok to me but would you recommend I do anything with this head? How can I test it or is that strictly a machine shop service? Do you recommend a particular machine shop? Do you do this work? I think the service manual has some sort of cyl head leak down bench test... have you ever done this? What about the mushroomed area in the aluminum? What would have caused this and how do I prevent the same from happening again?

In thinking about what caused all of this and how to prevent it from happening again. I have not found anything foreign inside the Engine. Just the piston pieces. Could the piston have hit a stuck intake valve and broke the chunks off maybe? I have cleaned the head, oiled it and have turned the cam several rotations to watch valve movement. They aren't sticking now, and look like they are moving there full travel up and down, but I do have some carbon and muck inside the guide areas. I was thinking that with heat, maybe a valve stuck.. is this a fair assumption and a typical scenario?

 

One way to tell off hand if the valves need attention is to pour solvent(kerosene,spray carb cleaner,etc) into the intake and exhaust ports. If any leakage they at least need to be lapped. You can have a machine shop do this usually at a reasonable price since you need to have a valve compression clamps valve grinding compound,etc and the desire.The shop can tell real quick if any valves are bent. Just my opinion,if you're going to spend this kind of money on rebuilding I'd split this engine as I've found plenty of metal shavings in the pump screen in the past when a piston shattered. Hard to get all the shavings out of the nooks and crannies just flushing cases.Plus "IF" mine I'd tear the oil pump apart and see if rotors are worn and need to be replaced/cleaned of any debris.Doing a partial job on this engine can just cost extra money if you're not careful.Plus check over the cam lobes closely as this model was bad about exhaust lobe wear. Check out the pump parts. Dirt Cheap Yamaha, Honda, Arctic Cat & Polaris OEM Parts & Accessories – Cycle Parts Warehouse
This pdf manual can get you familiar with the engine. http://gh-ftp.com/ORV%20Manuals/Pola...0%20-%20SM.pdf